Plastic bag and plastic bag making apparatus

ABSTRACT

A plastic bag including a pair of side gussets  1  is arranged to be free of leakage from one end portion of each of the side gussets. 
     Each of the side gussets has opposite end portions one of which is folded obliquely along a folding line as it is folded into halves so that a triangular flap  13  can be formed by the end portion to be shaped into a triangle having a base formed by the folding line. The triangle has an apex formed by an intersection between one of the opposite end edges and the folded inner edge of each of the side gussets. The panels  2 , the side gussets and the triangular flaps are heat sealed respectively along the opposite side edges  3  of panels.

TECHNICAL FIELD

The invention relates to a plastic bag and a plastic bag makingapparatus.

BACKGROUND

FIG. 18 illustrates a plastic bag of prior art. The plastic bag includesa pair of side gussets 1 which are incorporated into two superposedlayers of panels 2 having opposite side edges 3. The side gussets 1extend along the opposite side edges 3 of panels 2. In addition, each ofthe side gussets 1 is folded into halves and interposed between thelayers of panels 2 to have a folded inner edge 4 and open outer edges 5,as shown in FIG. 19 and FIG. 20. The panels 2 and the side gussets 1 areheat sealed with each other along the opposite side edges 3 of panels 2so that heat sealed portions 6 can be formed along the opposite sideedges 3 of panels 2. The plastic bag can therefore be enlarged by theside gussets 1 to obtain an increased capacity.

Furthermore, it has been recently attempted to make a fastener 7incorporated into the plastic bag including the side gussets 1. Thefastener 7 may be called a zipper and has been used generally in itself.It includes a male member formed integrally with a tape and fitted intoa female member which is also formed integrally with a tape. The maleand female members extend along ones of the opposite end edges 8 ofpanels 2 to be interposed between the layers of panels 2. The layers ofpanels 2 are heat sealed with the tapes of male and female members. Inaddition, the layers of panels 2 are heat sealed with each other alongthe opposite end edges 8 and 9 thereof so that heat sealed portions 6can be formed along the opposite end edges 8 and 9 of panels 2. Theplastic bag can therefore be opened and closed by the fastener 7 aftercutting the panels 2 along a cutting line 10 extending between the endedges 8 and the fastener 7.

In this case, the plastic bag is problematic in that each of the sidegussets 1 includes opposite end portions one of which must be intervenedagainst the fastener 7. It is therefore required to make the panels 2heat sealed with the end portion of each of the side gussets 1 near thefastener 7 in such a way that the panels 2 and the side gusset 1 can beclosed to each other by the heat sealed portion. However, it isdifficult to accomplish such the step with a sealing strength enough tobe free of leakage.

It is therefore an object of the invention to provide a plastic bagincluding a pair of side gussets, which is arranged to be free ofleakage from one end portion of each of the side gussets.

DISCLOSURE OF THE INVENTION

According to the invention, the plastic bag includes a pair of sidegussets which are incorporated into two superposed layers of panelshaving opposite side edges. The side gussets extend along the oppositeside edges of panels. In addition, each of the side gussets is foldedinto halves and interposed between the layers of panels to have a foldedinner edge, open outer edges and opposite end edges. Each of the sidegussets has opposite end portions one of which is folded obliquely alonga folding line as it is folded into halves so that a triangular flap canbe formed by the end portion to be shaped into a triangle having a baseformed by the folding line. The triangle has an apex formed by anintersection between one of the opposite end edges and the folded inneredge of each of the side gussets. The panels, the side gussets and thetriangular flaps are heat sealed respectively along the opposite sideedges of panels.

In a preferred embodiment, the folding line comprises a bisector whichreaches intersections between the end edge and the open outer edges ofeach of the side gussets to bisect an angle between the end edge and theopen outer edges of each of the side gussets. The triangular flap isshaped into an isosceles triangle having the base formed by the foldingline.

The triangular flap may have a folded side along which the triangularflap and the side gusset are heat sealed with each other.

Hole or notch means may be formed in the triangular flap so that one ofthe layers of panels can be heat sealed with the side gusset through thehole or notch means.

Furthermore, according to the invention, an apparatus is arranged tomake plastic bags. The apparatus includes feeding means of continuitiesof panels. Each of the continuities comprises panels continuous witheach other widthwise thereof. The continuities are superposed into twolayers and intermittently fed by the feeding means in a direction inwhich the panels are continuous with each other. The apparatus furtherincludes supply means of side gussets. Each of the side gussets ispreviously folded into halves to have a folded side edge. The sidegusset is then supplied to the continuity by the supply means wheneverthe continuities are intermittently fed so that the side gusset can beinterposed between the layers of continuities. The side gusset extendsperpendicularly to the feeding direction of continuities. The apparatusfurther includes folding means of side gussets. Each of the side gussetshas opposite end portions one of which is folded obliquely along afolding line by the folding means as it is folded into halves after orbefore the side gusset is supplied to the continuity so that atriangular flap can be formed by the end portion to be shaped into atriangle having a base formed by the folding line. The triangle has anapex formed by an intersection between one of the opposite end edges andthe folded side edge of each of the side gussets. The apparatus furtherincludes heat seal means by which the continuities, the side gusset andthe triangular flap are heat sealed respectively perpendicularly to thefeeding direction of continuities so that heat sealed portions can beformed perpendicularly to the feeding direction of continuities wheneverthe continuities are intermittently fed. The apparatus further includescutter means by which the continuities are cut along the heat sealedportions of continuities, side gusset and triangular flap whenever thecontinuities are intermittently fed, to make plastic bags each of whichcomprises two superposed layers of panels having opposite side edges,and a pair of side gussets extending along the opposite side edges ofpanels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a plastic bag according to the invention.

FIG. 2 is an explosive view of the plastic bag of FIG. 1.

FIG. 3 is an explanatory view of the side gusset of FIG. 2 before beingfolded obliquely along a folding line.

FIG. 4 is an explanatory view of the side gusset of FIG. 3 after beingfolded obliquely.

FIG. 5 is an explanatory view of the triangular flap of FIG. 4 afterbeing heat sealed.

FIG. 6 is an explanatory view of other embodiment.

FIG. 7 is an explanatory view of the side gusset of FIG. 6 after beingfolded obliquely.

FIG. 8 is an explanatory view of the triangular flap of FIG. 7 afterbeing heat sealed.

FIG. 9 is a plan view of other embodiment.

FIG. 10 is an explosive view of the plastic bag of FIG. 9.

FIG. 11 is a side view of an apparatus for making plastic bags accordingto the invention.

FIG. 12 is a plan view of the continuities of FIG. 11.

FIG. 13 is an explanatory view illustrating a process of making the sidegusset of FIG. 12.

FIG. 14 is an explanatory view illustrating a process of making the sidegusset next to that of FIG. 13.

FIG. 15 is an explanatory view illustrating a process of making the sidegusset next to that of FIG. 14.

FIG. 16 is an explanatory view of other embodiment.

FIG. 17 is an explanatory view illustrating a process of making the sidegusset next to that of FIG. 16.

FIG. 18 is a plan view of a plastic bag of prior art.

FIG. 19 is a sectional view of the plastic bag of FIG. 18.

FIG. 20 is an explosive view of the plastic bag of FIG. 18.

BEST MODE TO CARRY OUT THE INVENTION

FIG. 1 illustrates a plastic bag according to the invention. The plasticbag includes a pair of side gussets 1 which are incorporated into twosuperposed layers of panels 2 having opposite side edges 3, as in thecase of plastic bag of FIG. 18. The side gussets 1 extend along theopposite side edges 3 of panels 2. In addition, each of the side gussets1 is folded into halves and interposed between the layers of panels 2 tohave a folded inner edge 4 and open outer edges 5, as shown in FIG. 2.The panels 2 and the side gussets 1 are heat sealed with each otheralong the opposite side edges 3 of panels 2 so that heat sealed portions6 can be formed along the opposite side edges 3 of panels 2, as also inthe case of plastic bag of FIG. 18. Furthermore, a fastener 7 isincorporated into the plastic bag. The fastener includes a male memberformed integrally with a tape and fitted into a female member which isalso formed integrally with a tape. The male and female members extendalong ones of the opposite end edges 8 of panels 2 to be interposedbetween the layers of panels 2. The layers of panels 2 are heat sealedwith the tapes of male and female members, as also in the case ofplastic bag of FIG. 18. In addition, the layers of panels 2 are heatsealed with each other along the opposite end edges 8 and 9 thereof sothat heat sealed portions 6 can be formed along the opposite end edges 8and 9 of panels 2, as also in the case of plastic bag of FIG. 18. Theplastic bag can therefore be opened and closed by the fastener 7 aftercutting the panels 2 along a cutting lines 10 extending between the endedges 8 and the fastener 7.

Furthermore, each of the side gussets 1 has opposite end portions one ofwhich is folded obliquely along a folding line 11 near the fastener 7 asit is folded into halves so that a triangular flap 13 can be formed bythe end portion to be shaped into a triangle having a base formed by thefolding line 11, as shown in FIG. 3 and FIG. 4. The triangle has an apexformed by an intersection between one of the opposite end edges 12 andthe folded inner edge 4 of each of the side gussets 1. The triangularflap 13 has therefore a folded side 14 and open side 15. In addition,the panels 2, the side gussets 1 and the triangular flaps 13 are heatsealed respectively along the opposite side edges 3 of panels 2 so thatheat sealed portions 6 can be formed along the opposite side edges 3 ofpanels 2.

Accordingly, one of the layers of panels 2 is heat sealed with thetriangular flaps 13 along the opposite side edges 3 of panels 2, whilethe triangular flaps 13 are heat sealed with the side gussets 1 alongthe opposite side edges 3 of panels 2, when the panels 2, the sidegussets 1 and the triangular flaps 13 are heat sealed respectively alongthe opposite side edges 3 of panels 2 after the end portion of each ofthe side gussets 1 being folded obliquely. In addition, the other layerof panel 2 is heat sealed with the side gussets 1 along the oppositeside edges 3 of panels 2 on the opposite side to the triangular flaps13. The panels 2 and the side gussets 1 can therefore be closed to eachother by means of the heat sealed portions 6 with a sealing strengthenough to be free of leakage from the end portion of each of the sidegussets 1.

In the embodiment, the folding line 11 comprises a bisector whichreaches intersections between the end edge 12 and the open outer edges 5of each of side gussets 1 to bisect an angle α between the end edge 12and the open outer edges 5 of each of the side gussets 1. The triangularflap 13 is therefore shaped into an isosceles triangle having the baseformed by the folding line 11. The angle α is 90° so that the isoscelestriangle must be a right one. The end edge 12 is therefore aligned withthe open outer edges 5 of side gusset 1 when the end portion is foldedobliquely along the folding line 11. In addition, the triangular flap 13and the side gusset 1 are then heat sealed with each other along thefolded side 14 of triangular flap 13 so that a heat sealed portion 16can be formed along the folded side 14 of triangular flap 13, as shownin FIG. 5. It is then inserted between the layers of panels 2 to beinterposed between them. The panels 2, the side gussets 1 and thetriangular flaps 13 are then heat sealed respectively along the oppositeside edges 3 of panels 2, as described above. Furthermore, hole or notchmeans 17 is formed in the triangular flap 13 so that one of the layersof panels 2 can be heat sealed with the side gusset 1 through the holeor notch means 17.

Accordingly, the end edge 12 can be kept being aligned with the openouter edges 5 of each of the side gussets 1 to be conveniently heatsealed therewith. In addition, the triangular flap 13 and the sidegusset 1 can be closed to each other by means of the heat sealed portion16. Furthermore, the triangular flap 13 includes the open sides 15 whichcan be closed to each other by the panel 2 and the side gusset 1 heatsealed with each other through the hole or notch means 17.

In the plastic bag, each of the panels 2 and the side gussets 1comprises a laminated plastic film composed of a base material such asnylon and a sealant such as polyethylene or polypropylene which islaminated on the base material. The layers of panels 2 have innersurfaces formed by the sealant and outer surfaces formed by the basematerial. Each of the side gussets 1 has outer surfaces formed by thesealant and inner surfaces formed by the base material when being foldedinto halves. The triangular flap 13 has therefore outer surfaces formedby the sealant and inner surfaces formed by the base material.

It should therefore be recognized that the panels 2, the side gussets 1and the triangular flaps 13 can be heat sealed respectively along theopposite side edges 3 of panels 2 when being sandwiched between andheated and pressurized by a pair of heat seal bars. In this step, one ofthe layers of panels 2 is heat sealed with the triangular flaps 13 bythe sealant, while the triangular flaps 13 are heat sealed with the sidegussets 1 by the sealant, in the positions of triangular flaps 13. Theother layer of panel 2 is heat sealed with the side gussets 1 by thesealant on the opposite side to the triangular flaps 13. Each of theside gussets 1 has the inner surfaces not sealed with each other byreason that the inner surfaces are formed not by the sealant but by thebase material. In addition, the layers of panels 2 are heat sealed withthe side gussets 1 in positions other than the positions of triangularflaps 13 and heat sealed with each other and with the fastener 7 inpositions other than the positions of side gussets 1.

It is not always necessary that the triangular flap 13 is shaped intothe right triangle by keeping the angle α being 90° between the end edge12 and the open outer edges 5 of each of the side gussets 1. In otherembodiment shown in FIG. 6, the side gusset 1 includes the end portioncut obliquely as it is folded into halves. The end portion is thenfolded obliquely along a folding line 11 as it is folded into halves, asshown in FIG. 7. The folding line 11 comprises a bisector bisecting anangle α between the end edge 12 and the open outer edges 5 of each ofthe side gussets 1. A triangular flap 13 is shaped into an isosceleshaving a base formed by the folding line 11. In this case, the angle αis not 90° but more or less than that. The triangular flap 13 istherefore not shaped into a right one. The end edge 12 is neverthelessaligned with the open outer edges 5 of each of the side gussets 1 whenthe end portion is folded obliquely along the folding line 11.

The triangular flap 13 and the side gusset 1 are then heat sealed witheach other along the folded hypotenuse 14 of triangular flap 13 so thata heat sealed portion 16 can be formed along the folded side 14. Inaddition, the panels 2, the side gussets 1 and the triangular flaps 13are heat sealed respectively along the side edges 3 of panels 2 afterthe side gussets 1 are inserted between the layers of panels 2 to beinterposed between them. The side gusset 1 has the end edge 12 which canbe kept being aligned with the open outer edges 5 to be convenientlyheat sealed therewith at the position of triangular flap 13.

In other embodiment shown in FIG. 9, a bottom 18 is incorporated into aplastic bag including the side gussets 1 and the fastener 7. Each of theside gussets 1 includes the opposite end portions one of which is foldedobliquely as it is folded into halves so that the triangular flap 13 canbe formed by the end portion. In addition, the panels 2, the sidegussets 1 and the triangular flaps 13 are heat sealed respectively alongthe opposite side edges 3 of panels 2, as in the case of plastic bag ofFIG. 1.

Furthermore, each of the side gussets 1 includes the other end portionfolded at an angle of 45° so that an auxiliary gusset 19 can be formedby the other end portion, folded into halves and interposed between thelayers of side gusset 1, as shown in FIG. 10. The bottom 18 is foldedinto halves and interposed between the layers of panels 2 and betweenthe layers of auxiliary gussets 19. The auxiliary gussets 19 and thebottom 18 are heat sealed with each other along the opposite side edges3 of panels 2, while the panels 2 and the bottom 18 are heat sealed witheach other along the other end edges 9 of panels 2. The plastic bag cantherefore stably stand by the bottom 18 which can be flat when theplastic bag is enlarged.

In the plastic bag, the bottom 18 comprises a laminated plastic filmcomposed of a base material such as nylon and a sealant such aspolyethylene or polypropylene which is laminated on the base material,to have outer surfaces formed by the sealant and inner surfaces formedby the base material when being folded into halves. It should thereforebe recognized that the auxiliary gussets 19 and the bottom 18 can beheat sealed with each other by the sealant when being sandwiched betweenand heated and pressurized by the heat seal bars. In addition, thepanels 2 and the bottom 18 can be heat sealed with each other by thesealant when being sandwiched between and heated and pressurized by apair of heat seal bars. The bottom 18 has the inner surfaces not sealedwith each other by reason that the inner surfaces are formed not by thesealant but by the base material.

FIG. 11 illustrates an apparatus for making plastic bags of FIG. 1 orFIG. 9. The apparatus includes feeding means of continuities of panels2. The feeding means comprises feeding rollers 20 between which thecontinuities 2 are directed. Each of the continuities 20 comprisespanels continuous with each other widthwise thereof, as shown in FIG.12. The continuities 2 are superposed into two layers and intermittentlyfed by the feeding rollers 20 in a direction in which the panels arecontinuous with each other.

The apparatus further includes supply means of side gussets 1. Each ofthe side gussets 1 is previously folded into halves to have a foldedside edge 4. The side gusset 1 is then supplied to the continuity 2 bythe supply means whenever the continuities 2 are intermittently fed sothat the side gusset 1 can be interposed between the layers ofcontinuities 2. The side gusset 1 extends perpendicularly to the feedingdirection of continuities 2. In the embodiment, the side gusset isderived from continuity 1 having a double width. The continuity 1 ispreviously folded into halves on the opposite sides of the longitudinalcenterline 21 to have folded side edges 4. In addition, the continuity 1is directed perpendicularly to the feeding direction of one of thelayers of continuities 2 before the continuities 2 are superposed intotwo layers. The continuity 1 is then intermittently fed by the supplymeans such as conveyor belts to be supplied to and put on the continuity2 whenever the continuities 2 are intermittently fed. It should be notedthat the continuity 1 comprises side gussets continuous with each othertwo by two and longitudinally thereof. It should also be noted that thecontinuity 1 is intermittently fed by the supply means in a direction inwhich the side gussets are continuous with each other.

The continuity 1 includes hole or notch means 17 formed at the end edgethereof, as shown in FIG. 13. In addition, the continuity 1 is spotsealed with the continuity 2 to be provisionally attached thereto by aheat seal or ultrasonic seal apparatus after being supplied so that spotsealed portions 24 can be formed on the centerline 21 of continuity 1. Atriangular plate 22 is then put on the continuity 1 so that thecontinuity 1 can be held by the plate 22. The plate 22 is split into twopieces along the centerline 21 of continuity 1 to have oblique sides 23and a center slit. The continuity 1 is then cut by a cutter so that twoside gussets 1 can be separated and derived from the continuity to haveopposite end edges, as shown in FIG. 14. Hole or notch means 17 is againformed at the end edge of continuity 1 at the same time or after thecontinuity 1 is cut. The hole or notch means 17 may be previouslyformed. In this case, the continuity 1 is then cut at the position ofhole or notch means 17.

The apparatus further includes folding means of side gussets. Thefolding means comprises a spatula 25 which is moved perpendicularly tothe feeding direction of continuity 2 to be inserted between the sidegussets 1 and the continuity 2. Each of the side gussets 1 has oppositeend portions one of which is folded obliquely along the oblique side 23of plate 22 by the spatula 25 moved perpendicularly to the feedingdirection, as shown in FIG. 15. It should therefore be recognized thatthe end portion is folded obliquely along a folding line 11 by thespatula 25 as it is folded into halves after the side gusset 1 issupplied to the continuity 2 so that a triangular flap 13 can be formedby the end portion to be shaped into a triangle having a base formed bythe folding line 11. The triangle has an apex formed by an intersectionbetween one of the opposite end edges and the folded side edge 4 of eachof the side gussets 1. The triangular flaps 13 are then spot sealed withthe side gussets 1 to be provisionally attached thereto by a heat sealor ultrasonic seal apparatus so that spot sealed portions 24 can beformed on the longitudinal centerline 21 of side gussets 1. The plate 22is then moved perpendicularly to the feeding direction of continuity 2of panel to be extracted from the triangular flaps 13 and returned intothe original position. The spatula 25 is also moved and returned intothe original position.

Spatulas 26 may be moved parallel to the feeding direction of continuity2 to be inserted between the side gussets 1 and the continuity 2. Inthis case, each of the side gussets 1 has the end portion which isobliquely folded along the oblique side 23 of plate 22 by the spatulas26, as shown in FIG. 16 and FIG. 17. In addition, the triangular flaps13 are spot sealed with the side gussets 1 to be provisionally attachedthereto by a heat seal or ultrasonic seal apparatus. The plate 22 isthen extracted from the triangular flaps 13 and returned into theoriginal position. The spatulas 26 are also moved and returned into theoriginal positions.

The triangular flaps 13 and the side gussets 1 are then heat sealed witheach other along the folded hypotenuses 14 of triangular flaps 13 by aheat seal apparatus so that the heat sealed portions 16 can be formedalong the folded sides 14.

It is not always necessary that each of the side gussets 1 has the endportion which is folded obliquely as it is folded into halves after theside gusset 1 is supplied. The end portion may be folded obliquely as itis folded into halves at a side gusset making station, after the hole ornotch means 17 is formed in the end edge of side gusset 1 and before theside gusset 1 is supplied. The triangular flap 13 and the side gusset 1may also be heat sealed along the folded side 14. The side gusset 1 isthen fed by the supply means to be supplied to and put on the continuity2. The side gusset 1 is then provisionally attached to the continuity 2by the heat seal or ultrasonic seal apparatus.

The continuities 2 are then superposed into two layers at the positionof feeding rollers 20 by being intermittently fed so that the sidegussets 1 can be interposed between the layers of continuities 2. Theside gussets 1 extend perpendicularly to the feeding direction ofcontinuities 2. At the same time, continuity 7 is directed to andinserted between the layers of continuities 2. The continuity 7comprises fasteners continuous with each other longitudinally thereof.

The apparatus further includes heat seal means comprising a heat sealapparatus 27 by which the continuities 2 and the continuity 7 are heatsealed with each other whenever the continuities 2 are intermittentlyfed. The apparatus further includes heat seal means comprising a heatseal apparatus 28 by which the layers of continuities 2 are heat sealedwith each other along ones of the opposite end edges of continuities 2so that the heat sealed portion 6 can be formed along the end edges. Inaddition, the apparatus includes heat seal means comprising a heat sealapparatus 29 by which the continuities 2, the side gussets 1 and thetriangular flaps 13 are heat sealed respectively perpendicularly to thefeeding direction of continuities 2 and along the longitudinalcenterline 21 of side gussets 1 whenever the continuities 2 areintermittently fed so that the heat sealed portions 6 can be formedperpendicularly to the feeding direction of continuities 2 and along thelongitudinal centerline 21.

The apparatus further includes cutter means comprising a cutter 30 bywhich the continuities 2, the side gussets 1 and the continuity 7 arecut along the longitudinal centerline 21 of side gussets 2 and the heatsealed portions 6 of continuities 2, side gussets 1 and triangular flaps13 whenever the continuities 2 are intermittently fed, to make plasticbags each of which comprises two superposed layers of panels 2 havingopposite side edges 3, and a pair of side gussets 1 extending along theopposite side edges 3 of panels 2.

The cutter 30 may be actuated two times so that the continuities 2, theside gussets 1 and the continuity 7 can be cut on the opposite sides ofthe longitudinal centerline 21 of side gussets 1 by the cutter 30. Thecutter 30 may include two THOMSON blades spaced from each other at asmall distance and parallel to the feeding direction of continuities 2so that the continuities 2, the side gussets 1 and the continuity 7 canbe cut on the opposite sides of longitudinal centerline 21 of sidegussets 1. Other blades may be spaced from each other at a smalldistance so that the continuities 2, the side gussets 1 and thecontinuity 7 can be cut on the opposite sides of longitudinal centerline21 of side gussets 1.

The apparatus can therefore make the plastic bags of FIG. 1.

In connection with the plastic bag of FIG. 9, the side gussets 1 may befolded by appropriate means so that the auxiliary gussets 19 can beformed by the other end portions of side gussets 1 in the apparatus ofFIG. 11. In addition, a bottom 18 may be previously folded into halvesand then inserted and interposed between the layers of continuities 2and the layers of auxiliary gussets 19. The auxiliary gussets 19 and thebottom 18 are then heat sealed with each other along the opposite sideedges 3 of panels 2. In addition, the panels 2 and the bottom 18 areheat sealed along the other end edges 9 of panels 2, to make plasticbags of FIG. 9. The auxiliary gussets 19 may be formed by the other endportions of side gussets 1 at the side gusset making station when thehole or notch means 17 is formed and the triangular flap 13 is formedand heat sealed. The side gussets 1 are then supplied to and put on thecontinuity 2.

According to the invention, the triangular flaps 13 are formed by theend portions of side gussets 1 folded obliquely, described above. Thepanels 2, the side gussets 1 and the triangular flaps 13 are then heatsealed respectively along the opposite side edges 3 of panels 2. Thepanels 2 and the side gussets 1 can therefore be closed to each otherwith a sealing strength enough to be free of leakage from the endportion of each of the side gussets 1.

1. A plastic bag comprising two superposed layers of panels havingopposite edges, and a pair of side gussets extending along the oppositeside edges of panels, each of the side gussets being folded into halvesand interposed between the layers of panels to have a folded inner edge,open outer edges and opposite end edges, each of the side gussets havingopposite end portions one of which is folded obliquely along a foldingline as it is folded into halves so that a triangular flap can be formedby the end portion to be shaped into a triangle having a base formed bythe folding line, the triangle having an apex formed by an intersectionbetween one of the opposite end edges and the folded inner edge of eachof the side gussets, the panels, the side gussets and the triangularflaps being heat sealed respectively along the opposite side edges ofpanels, each of the panels and the side gussets comprising a laminatedplastic film composed of a base material and a sealant which islaminated on the base material, the layers of panels having innersurfaces formed by the sealant and outer surfaces formed by the basematerial, each of the side gussets having outer surfaces formed by thesealant and inner surfaces formed by the base material when being foldedinto halves, the triangular flaps having outer surfaces formed by thesealant and inner surfaces formed by the base material, the panels andthe side gussets being heat sealed with each other by the sealant alongthe opposite side edges of panels so that heat sealed portions can beformed along the opposite side edges of panels, the triangular flapshaving open sides extending along the opposite side edges of panelspositioned in the heat sealed portions, each triangular flap being heatsealed with one of the layers of panels by the sealant along theopposite side edges of panels, while the triangular flaps are heatsealed with the side gussets by the sealant.
 2. The plastic bag as setforth in claim 1 wherein the triangular flap has a folded side alongwhich the triangular flap and the side gusset are heat sealed with eachother.
 3. The plastic bag as set forth in claim 1 wherein hole or notchmeans is formed in the triangular flap so that one of the layers ofpanels can be heat sealed with the side gusset through the hole or notchmeans.
 4. The plastic bag as set forth in claim 1 wherein the foldingline comprises a bisector which reaches intersections between the endedge and the open outer edges of each of the side gussets to bisect anangle between the end edge and the open outer edges of each of the sidegussets, the triangular flap being shaped into an isosceles trianglehaving the base formed by the folding line.
 5. The plastic bag as setforth in claim 4 wherein the triangular flap has a folded side alongwhich the triangular flap and the side gusset are heat sealed with eachother.
 6. The plastic bag as set forth in claim 4 wherein hole or notchmeans is formed in the triangular flap so that one of the layers ofpanels can be heat sealed with the side gusset through the hole or notchmeans.
 7. The plastic bag as set forth in claim 2 wherein hole or notchmeans is formed in the triangular flap so that one of the layers ofpanels can be heat sealed with the side gusset through the hole or notchmeans.